/*
Copyright (C) 2021 by clash authors

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, version 3.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

package obfs

import (
	"bytes"
	"crypto/rand"
	"encoding/binary"
	"io"
	"net"
	"time"

	"libcore/clash/common/pool"
)

const (
	chunkSize = 1 << 14 // 2 ** 14 == 16 * 1024
)

// TLSObfs is shadowsocks tls simple-obfs implementation
type TLSObfs struct {
	net.Conn
	server        string
	remain        int
	firstRequest  bool
	firstResponse bool
}

func (to *TLSObfs) read(b []byte, discardN int) (int, error) {
	buf := pool.Get(discardN)
	_, err := io.ReadFull(to.Conn, buf)
	pool.Put(buf)
	if err != nil {
		return 0, err
	}

	sizeBuf := make([]byte, 2)
	_, err = io.ReadFull(to.Conn, sizeBuf)
	if err != nil {
		return 0, nil
	}

	length := int(binary.BigEndian.Uint16(sizeBuf))
	if length > len(b) {
		n, err := to.Conn.Read(b)
		if err != nil {
			return n, err
		}
		to.remain = length - n
		return n, nil
	}

	return io.ReadFull(to.Conn, b[:length])
}

func (to *TLSObfs) Read(b []byte) (int, error) {
	if to.remain > 0 {
		length := to.remain
		if length > len(b) {
			length = len(b)
		}

		n, err := io.ReadFull(to.Conn, b[:length])
		to.remain -= n
		return n, err
	}

	if to.firstResponse {
		// type + ver + lensize + 91 = 96
		// type + ver + lensize + 1 = 6
		// type + ver = 3
		to.firstResponse = false
		return to.read(b, 105)
	}

	// type + ver = 3
	return to.read(b, 3)
}

func (to *TLSObfs) Write(b []byte) (int, error) {
	length := len(b)
	for i := 0; i < length; i += chunkSize {
		end := i + chunkSize
		if end > length {
			end = length
		}

		n, err := to.write(b[i:end])
		if err != nil {
			return n, err
		}
	}
	return length, nil
}

func (to *TLSObfs) write(b []byte) (int, error) {
	if to.firstRequest {
		helloMsg := makeClientHelloMsg(b, to.server)
		_, err := to.Conn.Write(helloMsg)
		to.firstRequest = false
		return len(b), err
	}

	buf := pool.GetBuffer()
	defer pool.PutBuffer(buf)
	buf.Write([]byte{0x17, 0x03, 0x03})
	binary.Write(buf, binary.BigEndian, uint16(len(b)))
	buf.Write(b)
	_, err := to.Conn.Write(buf.Bytes())
	return len(b), err
}

// NewTLSObfs return a SimpleObfs
func NewTLSObfs(conn net.Conn, server string) net.Conn {
	return &TLSObfs{
		Conn:          conn,
		server:        server,
		firstRequest:  true,
		firstResponse: true,
	}
}

func makeClientHelloMsg(data []byte, server string) []byte {
	random := make([]byte, 28)
	sessionID := make([]byte, 32)
	rand.Read(random)
	rand.Read(sessionID)

	buf := &bytes.Buffer{}

	// handshake, TLS 1.0 version, length
	buf.WriteByte(22)
	buf.Write([]byte{0x03, 0x01})
	length := uint16(212 + len(data) + len(server))
	buf.WriteByte(byte(length >> 8))
	buf.WriteByte(byte(length & 0xff))

	// clientHello, length, TLS 1.2 version
	buf.WriteByte(1)
	buf.WriteByte(0)
	binary.Write(buf, binary.BigEndian, uint16(208+len(data)+len(server)))
	buf.Write([]byte{0x03, 0x03})

	// random with timestamp, sid len, sid
	binary.Write(buf, binary.BigEndian, uint32(time.Now().Unix()))
	buf.Write(random)
	buf.WriteByte(32)
	buf.Write(sessionID)

	// cipher suites
	buf.Write([]byte{0x00, 0x38})
	buf.Write([]byte{
		0xc0, 0x2c, 0xc0, 0x30, 0x00, 0x9f, 0xcc, 0xa9, 0xcc, 0xa8, 0xcc, 0xaa, 0xc0, 0x2b, 0xc0, 0x2f,
		0x00, 0x9e, 0xc0, 0x24, 0xc0, 0x28, 0x00, 0x6b, 0xc0, 0x23, 0xc0, 0x27, 0x00, 0x67, 0xc0, 0x0a,
		0xc0, 0x14, 0x00, 0x39, 0xc0, 0x09, 0xc0, 0x13, 0x00, 0x33, 0x00, 0x9d, 0x00, 0x9c, 0x00, 0x3d,
		0x00, 0x3c, 0x00, 0x35, 0x00, 0x2f, 0x00, 0xff,
	})

	// compression
	buf.Write([]byte{0x01, 0x00})

	// extension length
	binary.Write(buf, binary.BigEndian, uint16(79+len(data)+len(server)))

	// session ticket
	buf.Write([]byte{0x00, 0x23})
	binary.Write(buf, binary.BigEndian, uint16(len(data)))
	buf.Write(data)

	// server name
	buf.Write([]byte{0x00, 0x00})
	binary.Write(buf, binary.BigEndian, uint16(len(server)+5))
	binary.Write(buf, binary.BigEndian, uint16(len(server)+3))
	buf.WriteByte(0)
	binary.Write(buf, binary.BigEndian, uint16(len(server)))
	buf.Write([]byte(server))

	// ec_point
	buf.Write([]byte{0x00, 0x0b, 0x00, 0x04, 0x03, 0x01, 0x00, 0x02})

	// groups
	buf.Write([]byte{0x00, 0x0a, 0x00, 0x0a, 0x00, 0x08, 0x00, 0x1d, 0x00, 0x17, 0x00, 0x19, 0x00, 0x18})

	// signature
	buf.Write([]byte{
		0x00, 0x0d, 0x00, 0x20, 0x00, 0x1e, 0x06, 0x01, 0x06, 0x02, 0x06, 0x03, 0x05,
		0x01, 0x05, 0x02, 0x05, 0x03, 0x04, 0x01, 0x04, 0x02, 0x04, 0x03, 0x03, 0x01,
		0x03, 0x02, 0x03, 0x03, 0x02, 0x01, 0x02, 0x02, 0x02, 0x03,
	})

	// encrypt then mac
	buf.Write([]byte{0x00, 0x16, 0x00, 0x00})

	// extended master secret
	buf.Write([]byte{0x00, 0x17, 0x00, 0x00})

	return buf.Bytes()
}
